Programmable logic on integrated circuits was introduced and popularized during the 1970's. Programmable array logic (PAL) and programmable logic devices (PLD) utilized advanced semiconductor processing technology, and enabled customers to purchase standard off-the shelf circuits that were essentially blank, and customize the circuits at the customer location. Unfortunately, the small density of these circuits limited the size and complexity of the designs.
Programmable elements, such as fuses, are well known for use in semiconductor devices, such as PLDs. See, for example, "Advanced Single Poly BiCMOS Technology for High Performance Programmable TTL/ECL Applications" by Iranmanesh, et al. IEEE 1990 Bipolar Circuits and Technology Meeting. Semiconductor fuse materials include polysilicon, Ti/W and Pt/Si (see U.S. Pat. No. 4,796,075).
In addition to PALs, PLDs and fuses, Application Specific Integrated Circuits (ASICs) can be used to implement custom logic. The ASICs market exploded during the 1980's with the popularization of the masked gate array. A masked gate array employs a standard base array which is stored in inventory and metallized in accordance with the needs of a particular customer, to form a desired logic circuit. However, a masked gate array does not give system designers the flexibility and time-to-market advantage of the PLDs and PALs which are field programmable.
This limitation of ASICs fostered the birth of Field Programmable Gate Arrays (FPGAs) in the early to mid 1980's. Integrated circuit (IC) programmable logic is built using programmable elements such as, for example, SRAMs (static random access memory), EPROMs (electrically programmable read only memory), fuses and antifuses. Antifuse materials typically used in the semiconductor field include silicon oxide/silicon nitride composites (see, for example, U.S. Pat. Nos. 4,823,181, 4,876,220 and 5,258,643), and amorphous silicon (see, for example, U.S. Pat. Nos. 4,914,055 and 5,196,724). Properties and structures of such antifuse materials are described in the prior art (see, for example, "Dielectric Based Antifuse for Logic and Memory ICs" by Hamdy, et. al., 1988 International Electronic Devices Meeting (IEDM); "Antifuse Structure Comparison for Field Programmable Gate Arrays" by Chiang, et. al. 1992 IEDM; "Interconnect Devices for Field Programmable Gate Array" by Hu, 1992 IEDM; "Conducting Filament of the Programmed Metal Electrode Amorphous Silicon Antifuse" by Gordon, et. al. 1993 IEDM).
Several attempts have also been made to build programmable printed circuit boards (PCBs) and multichip modules (MCMs) as described in, for example, U.S. Pat. Nos. 5,321,322, 5,311,053, 5,055,973, 5,077,451, 4,458,297 and 4,847,792. Programmable printed circuit boards can be built utilizing programmable ICs (silicon chips made using semiconductor technology) mounted on top of a PCB at key locations throughout the PCB as described in U.S. Pat. Nos. 5,055,973, 5,077,451 and Aptix Data Book (Feb. 1993). The Aptix Data Book is available from Aptix Corporation, 225 Charcot Avenue, San Jose, Calif. 95131. However, switches inside a programmable IC are highly resistive in comparison to switches directly on a substrate.
In U.S. Pat. Nos. 4,458,297 and 4,847,792, a silicon circuit board (SCB), that includes programmable switches made of amorphous silicon material, has silicon devices mounted on the top. However, the amorphous silicon switches in the silicon circuit board are fabricated by semiconductor technology.
U.S. Pat. No. 4,652,974 to Ryan describes a method and structure for effecting engineering changes in a multiple device module package. However, Ryan's deletable connection 36 is part of a device 14 that has a "tailorable metallurgy system" (column 4, line 67), is "joined to the substrate using solder joining techniques to join the respective solder pads" (column 5, lines 7-9), and is an integrated circuit device (col. 6, line 22) mounted on top of a substrate that "is typically a multilayer ceramic substrate" (col. 3, lines 59-60). Therefore, Ryan's "deletable connection 36" is also fabricated by semiconductor technology. Although Ryan describes other deletable links, such as deletable line portion 17, Ryan suggests that such links are "severed with a laser beam" (column 4, line 51).
For ceramic packages, methods of making engineering change contact pads on a top surface have been described in, for example, U.S. Pat. No. 4,840,924 to Kinbara. Such engineering change contact pads are cut by mechanical or laser cutting and have connection conductor portions for manually connecting a wire to make a wiring change.
Passive programmable elements and architecture are disclosed in U.S. Pat. Nos. 5,321,322 and 5,311,053. However, both patents address a specific architecture with a fuse and an antifuse connected in series to form one element. (U.S. Pat. No. 5,321,322, col. 3, lines 65-67, and col. 4, lines 47-48 and U.S. Pat. No. 5,311,053, col. 3, lines 39-42).
The above approaches for programmable PCBs, SCBs and MCMs have several drawbacks. For example, using ICs (silicon chips) in the above products results in a high cost. Additionally, the speed of such products is degraded as routing of signals in and out of a silicon chip is not the most efficient way of making connections. Also if the programmable elements are fuses that have to be severed by laser or mechanical cutting, the programmable elements must be on the top surface of a PCB or MCM. Such top surface mounting takes up precious outer layer board space that could be used to add additional electronic components and/or circuit traces.
Programmable elements described above can be used in a socket for connecting an integrated circuit to a printed circuit board. For example, U.S. Pat. No. 4,609,241 to Peterson discloses a "programmable programmed" socket that includes a programmable device, such as an EPROM made of semiconductor technology. Therefore, such sockets are more expensive than and larger than a conventional socket by the cost and size of Peterson's "solid state electronic programmable device" (column 2, lines 63-64).
Programmable elements can also be used in a cable. U.S. Pat. No. 5,144,567 to Oelsch et al. discloses "a programmable plug and cable for computer keyboards" (title). The keyboard plug has encoder electronics that "comprise as the programmable IC a microcomputer 10 with integrated EPROM" (column 3, lines 42-46), which also increases the cost and size of the cable.
Programmable elements can also be used in a shorting plug. For example, U.S. Pat. No. 4,090,667 to Crimmins discloses a shorting plug with a number of electrically conductive bridges that electrically short terminal pins and that include a removable "removal portion". The removal portion is removed by a simple hand tool (col. 4, line 36) or by punches programmed to remove several removal portions in a single action (column 4, lines 60-61). However, Crimmins removal portions are removed mechanically, which is slow, cumbersome and tedious. Also a mechanical switch is usually large in size, compared to an electrically programmable switch. Finally, mechanical switches must be mounted on a top surface of a PCB and so take up precious board space, as noted above.
Semiconductor fuses can be electrically programmed, for example, by a method of U.S. Patent in which "column 180-1 is selected by drivers 130-1 through 130-N" (col. 2, lines 41-42) and "programming current is passed through the selected fuse device connected between column 180-1 and the selected row, thus opening that fuse device" (col. 2, lines 49-50). However, such a method requires "fusing driver array 105" that "includes a set of fusing drivers associated with each column" (col. 2, line 16-18) and so results in programming time of quadratic complexity, depending on the number of fuses to be programmed.
Therefore a new approach is necessary to provide low cost and fast time-to-market products that can be electrically programmed.